Electric control system



Oct. 23, 1945. E. .E. MOYER 2,387,601

ELECTRIC CONTROL SYSTEM Filed May 2, 1942 I I I 1 Inventor-z 1 Elmo'E. M05 P,

His Aotor n ey- Patented Oct. 23, 1945 ELECTRIC CONTROL SYSTEM Elmo E. Moyer, Scotia, eral Electric Compan York N. Y., assignor to Geny, a. corporation of New Application May 2, 1942, Serial No. 441,450

I tric valve control system for an alternating our- 17 Claims.

My invention relates to an electric control system and more particularly to a controlsystem for dynamo-electric machines, and to electric valve control circuits.

Electric valve equipment has been found desirable in the control of dynamo-electric machines, such as motors and generators, due to the great precision of control provided thereby. In accordance with the teachings of my invention described hereinafter, I provide a new and improved electric valve or electric discharge control circuit wherein a dynamo-electric machine, such as an alternating current synchronous motor, is controlled.

It is an object of my invention to provide a new-and improved electric control system.

It is another object of my invention to provide a new and improved electric valve control system for dynamo-electric machines.

It is a further object of my invention to provide a new and improved electric control system fora dynamo-electric machine such as an alternating current motor of the synchronous type.

It is a still further object of my invention to provide a new and improved electric valve control system for an alternating current motor of the synchronous type and which provides for control of the load imposed on the motor during the starting operation.

It is .a still further object of my invention to provide a new and improved electric valve control system for an alternating current synchronous inotor which drives a load device, through means such as a magnetic clutch, and in which the output speed of the load device is controlled.

Briefly stated, in the illustrated embodiment of my invention I provide a new and improved electric valve system for an alternating current syn chronous motor which drives a load device through a magnetic clutch. The magnetic coupling of the clutch is controlled in response to an operating condition of a driven load device. The electric valve control system not only maintains an operating condition, such as the speed of the load device at a substantially constant value, but also serves to limit the load imposed on the motor during starting and running conditions.

For a better understanding of my invention, reference may be had to the single figure of the drawing taken in connection with the accompanying description and its scope will be pointed out in the appended claims. The single figure of the drawing diagrammatically illustrates an embodiment of my invention as applied to an elecrent synchronous motor which drives a load device, such as a fan, through amagnetic clutch.

Referring now to the single figure of the drawing, I have there illustrated my invention as applied to a system for controlling a dynamoelectric machine which may be an alternating current synchronous motor I comprising armature windings 2 and a field winding 3. I provide starting means for the synchronous motor I. This starting means may be any of the conventional arrangements, now well known in the art, for starting a synchronous motor, and for the purposes of illustration I have chosen to represent the starting means as comprising a source of reduced voltage 4 which may be connected to the armature windings 2 by means of a circuit controlling means or switch 5. After the speed of the synchronous motor I has attained a predetermined value, the switch 5 is opened and switch 6 is closed, thereby connecting the armature windings 2 to an alternating current circuit I of normal operating. voltage.

The rotating member of the synchronous motor I is connected to load means which may cornprise a magnetic clutch 8 having relatively rotatable members 9 and I0, the former of which may be connected to the rotating member of the synchronous motor I through a shaft I I, and the latter of which may be connected to a load device, such as a fan I2, through a shaft I3. The magnetic clutch 8 is provided with a control winding M, the energization of which determines the relative rotation of the members 9 and I0 and, hence, controls the speed of the fan I2. In other words, the energization of the control winding I4 determines the magnetic flux linking the members 9 and Ill, and thereby determines the slip between these two members.

I provide current supply means I5 for energizing the control winding I4 of the magnetic clutch 8. This current supply electric valve rectifier which is energized from a suitable alternating current circuit I6 and which is connected to the control winding I4 through a circuit I'I. Although not limited thereto, I have chosen to represent the rectifier of the current supply means I5 as constituting a quarter phase rectifying system in which a transformer I8 is provided with primary and secondary windings arranged in the conventional Scott or T- connection. Primary winding means I9 may be connected to the alternating current circuit I6 through a circuit controlling means, such as a contactor 20 having contacts 2I23, inclusive.

' across the control winding i l.

Secondary windings 2 3 are comected to circuit ll through electric valve means a e, inclusive, If desired, switches 29 may be connected between the contacts 2 9-23 of contactor 2d and the alter= nating current circuit l6.

Electric valve means 25-2t, inclusive, are prei= erabl of the type employing an ionizable medium, such as a gas or a vapor capable of supporting an arc discharge, and each comprises an anode 3E1, a cathode 3i, and a control member such as a control grid 32 which determines the amount of current conducted thereby. Cathode heating elements 33, ii employed, may be energizecl'irom a suitable source of current 34. In some applications it may be desirable to employ electric valves having shield grids 35 which may be connected directly to the cathodes 3i. Tran= sient absorbing capacitances 36 may be connected in the manner illustrated to prevent the impression of extraneous transient voltages on control grids 32.

I also employ an excitation circuit 37! for sup plying to the control grids of electric valve means fit-28 an alternating component of grid potential. This portion of the control circuit for the electric valve means transformer 38 having primary windings 39 and secondary windings do, which are also connected in a Scott or T-connection and are phased with respect to the associated anode voltages to have a 120 electrical degree lagging relationship. The windings iii are connected to the respective associated grids through current limiting resistances ii. I also provide suitable voltage limiting means, such as a resistance 52, connected across the output circuit of the rectifier and connected Ihis resistance maybe oi the type having a negative impedance vcurrent characteristic such as that disclosed and claimed in United States Letters Patent No. 1,822,742, granted September 8, 1931 upon an application of Karl E. McEachron. Resistance 32 serves to prevent excessive voltage transients across winding Ml in the event the electric valve means fail to conduct current at a time when considerable energy is stored in winding id.

The output of the current supply means or the conductivity of the electric valve means 25-28 is controlled by means of a control circuit comprising conductors 3-3 and M. The magni= tude of the voltage provided by this control cir= cult controls the phase position or the resultant voltage impressed on control grids of electric valve means lib-Ed, and, hence, controls the time during the respective positive half cycles of applied anode-cathode voltage at which the electric valve means begin to conduct current.

I provide a direct current circuit including a positive conductor d anda negative conductor 436. This circuit may be energized from the alter nating current circuit it through a transformer ill comprising primary windings 33 and secondary windings 69, the latter of which may be provided with an electrical neutral connection which is connected to the negative conductor cc. Dia metric windings of the secondary windings constitute, with unidirectional conducting devices or electric valves hi and 52, a pair 01 hl phase rectifiers for energizing the direct current circuit comprising conductors {l5 and it. Smoothing re= 'actances 53 and 5d are connected between the electric valves ill and 52 and the positive conductor 55. It will be observed that by virtue or this connection continuity of energiration of the ze -2e may include a direct current circuit is provided, even though one rectifier circuit fails.

load device at a predetermined constant value. In the embodiment of my invention illustrated, I provide means for maintaining an operating condition such as the speed of the fan l2 constant. In addition, I also provide armature current limiting means or armature current preset means for controlling the maximum value of armature current of the motor l during the starting and running operations.

The control means which controls the voltage difference appearing between conductors it and id and which controls the current'transmitted by electric valve means 25-28 may comprise an electric discharge device provided with a grid 56. The discharge device 55, which is preferably of the high vacuum type, may be connected in series relation with a resistance ill and a constant voltage reference means, such as a glow discharge valve 58, which when in a conducting condition parallel with the discharge device 55, and a second glow discharge valve iii may be connected in parallel with the glow discharge valve 58. Electric discharge devices 55 and 59 conduct variable amounts of current through resistance 5i, thereby controlling the potential of conductor l l relative to conductor 43 and consequently controlling the voltage impressed on control grids 32 of electric valve means 25-28, inclusive.

Control electric discharge devices 52 and 63 provided with grids so may be employed for controlling the potential of grids 56 and to of electric discharge devices 55 and 59. The anode-cathode circuits of the control discharge devices 52 and til'are in parallel and are connected in series relation with a resistance 65 across conductors 35 and at. The control discharge devices 62 and ts transmit variable amounts of current through resistance as and consequently control the potential impressed on grids 56 and to of discharge devices 55 and 59. The potential of the cathodes oi the electric discharge devices 55 and 59 may be maintained at a substantially constant value, below the potential of conductor 35, by means of glow discharge valves 65 and Gil, which also may be connected in parallel in order to assure continuity of service.

As a mean for deriving a control voltage .which is responsive to a predetermined operat= iii sistances and i l.

' juncture of resistances l2, l3, and ill, and the cathodes of discharge devices 62 and In this manner the rate of variation of the potential impressed on control grids to of discharge de= vices 62 and 63 is controlled to prevent huntin or overshooting of the fan speed.

In order to efiect a rapid build-down of the current in the control winding I4 when the system indicates a rapid deenergization of control winding I4, I provide means for controlling the electric valves 25-28, inclusive, to act as an inverter, wherein the decrement of the electromagnetic energy stored in control winding I4 is transferred to alternating current circuit I5. This feature of control is disclosed and broadly claimed in United States Letters Patent No. 2,239,429, granted April 22, 1941, upon my application and which is assigned to the assignee of the present application. More particularly, I provide electric discharge devices 16 and I1 which may be connected in parallel. It is to be understood that a single discharge device wouldperform the function satisfactorily and that the two discharge devices are employed in order to assure continuity of service. Each of the discharge devices I6 and TI is provided with a control grid 10 which I is energized by a voltage which is responsive to the magnitude and polarity of the unidirectional voltage appearing across conductors 43 and 44. This control voltage may be obtained by means of a voltage divider comprising serially connected resistances I9, 80 and 8| which are connected between the positive conductor 45 of the direct current circuit and the upper terminal of the glow discharge valves 58 and GI. Resistance 80 is provided with an adjustable contactor 82 which controls the time at which the electric discharge devices I6 and I1 initiate the inversion operation.

Armature current limit means or armature current pre-set, means 85 are provided to limit the eifective range of operation of electric discharge devices 55 and 59. The armature current preset meansmay comprise electric discharge devices 81 and 88 having grids 89. The discharge devices 81 and 88 are employed as'apair to assure continuity of service, and only one device is nec- 'essary in order to carry out the function. The

discharge devices 81 and 88 are also connected in parallel with the electric discharge devices 55 and 59 and thereby limit the anode-cathode voltagethereof by conducting different amounts of current. v

The potential impressed on grids 89 of electric .discharge devices 01 and 88 is controlled by means of a control circuit including conductors 90 and iii. The grids 89 are also connected to grids56- and of electric discharge devices and 59 through an anti-hunting or damping circuit includin a capacitance 82 and resistances I 93 and 94. -When the voltage of the circuit comconductivity of electric valve means 25-28 to a value which will not cause excessive current to be consumed by the motor I.- The capacitance 92 operates as a damping or anti-hunting means,

- thereby controlling the rate ofapplication of voltage to grids 56 and of electric discharge de- .vices. .55 and 59 A further damping and filter circuitcomprising parallel connected resistance and capacitor may be connected across conductors 90 and EH to limit the eii'ective rate of change of the potential difierencetherebetween.

I provide, in the armature current preset means 86, means for impressing on the circuit including conductors 9 0 and BI a unidirectional voltage the magnitude of which varies in response to the current of the polyphase alternating current circuit 1. This latter means comprises a pair of biphase rectifiers energized in accordance with the currents of different phases of 'the polyphase circuit 1 and each comprises a I03 and I04 are connected between the positive conductor 90 and valves IOI and ,I02.

Voltage limiting means, such as resistances I05 and I06, may be connected across secondary windings 99 of the transformers, and may be of a material such as that described'in the above mentioned McEachron patent. I also connect across the secondary windings 99 voltage controlling or adjusting means which may comprise voltage dividers including resistances I01 having mechanically coupled adjustable contacts I08,.so that upon adjustment of one means the other is automatically adjusted to the same value thereby assuring the desired current limit or current preset if one of the biphase rectifiers fails;

I provide means for presetting the initial current ,transmitted by the current supply means I5, thereby controlling the rate of application of load to the synchronous motor I during the starting and running operation thereof. In this manner, the load is applied gradually to the motor. -One way in which this gradual application of load to motor I may be efiec'ted is by limiting the maximum conductivity-of the electric valve means 25-20 and, hence, limiting the maximum initial energization of control winding I4. More particularly, the initial energization of the winding I4 may be limited by biasing the control electric dischargd'devices to a predetermined initial degree of conductivity.

One way in which the desired initial presetting of the system may be provided is by interlocking the contactor 20 with the portion of the control system including the electric discharge devices.

For example, the contactor 20 may be provided with contacts I09 and H0 which are connected rent conducted by supply means I5 is maintained at a predetermined value until the synchronous motor I attains the desired predetermined speed at which the field winding 3 maybe energized. I provide means, such as a frequency responsive relay H2 having an actuating coil H3 and contacts H4 and H5. Actuating coil H3 may be energized from a circuit I I6 which is energized in response to the frequency or the currents induced in the field winding 3 or the synchronous motor I, so that relay H2 is energized to close contacts H5 when the speed attains a predetermined value. Upon closure of contacts H5, unidirectional current is transmitted to the field winding 3, thereby causing the motor Ito step into synchronism. Contacts II4 are-connected in circuit with contacts H of contactor 20 so that the initial conductivity of the electric valve means 25--28 is maintained until the field winding 3 is energized, or until the speed of the motor I has attained a value which permits synchronous operation thereof.

Actuating coil II! of contactor 20 is arranged to be energized from a suitable source of control current H8 through apparatus described immediately hereinafter. In order to prevent connection of the electric valve means 2528, inclusive, to the alternating current circuit I6 for a predetermined time to permit the cathodes thereof to assume a safe operating temperature, I employ a time delay relay H9 havingcontacts I20 connected in series relation with the actuating coil HI. Contactor 20 is also provided with sealing-in contacts I2.Ia. An auxiliary starting switch =I2I may be connected in series relation with the actuating coil H'I, but

is not effective of course until the actuating coil III has been energized by apparatus described hereinafter. A stop circuit controlling means, such as a stop switch I22, is connected in circuit with doll H1 and this means is, of course, effective upon actuation at all times to discon nect the current supply means I from the alternating current circuit I B.

I provide control circuits I23 and I24 which 'may be physically displaced from each other and which may constitute remote control stations for pre-establishing the initial load-imposed on synchronous motor I and for presetting the speed or controlling the speed of the Ian I 2. Circuits I23 and I24 each comprise a voltage divider including a resistance I25 which is connected,

through switch I21 in parallel with valves 58 and SI, and through switch I28 the contact I30 of which is connected to circuit H to control electric discharge devices 62 and 63. More particularly, theresistances I25 are connected to the upper or positive terminals 01 glow discharge revices 58 and BI through an adjustable means, such as an adjustable resistance I28, the function of which will be explained hereinafter. Other terminals of resistance I25 are connected to the negative terminals of the glow discharge devices 58 and GI through a conductor I29. Each of the control circuits I 23 and I24 is provided with an adjustable contact I30 the position of which determines the speed of the fan I2 and which consequently, in order to obtain this effect, determines the conductivity of the electric discharge devices 55, 59, 62 and 63. As a means for presetting these discharge devices so that the system is in a condition which will not impose a severe starting load on the synchronous motor I, the contacts I30 are provided with interlockingfcontacts I3I which prevent energization of the gactuating coil II! of contactor 20, unless the electric discharge devices55, 59, 82 and 63 are setito establish the proper starting condition. Adjustable contacts I30 are connected to the positiige conductor of circuit II through a conductor I3;2. A resistance I33 is connected between con-- ductor I32 and'the negative conductor-48 of the direct current circuit. Resistance I28 determines the maximum speed oi. the fan I2 when contact, arm I30 is moved to the switch end I210! resistance I25.

The operation of the embodiment of my invention shown in the single figure of the drawing will be explained by considering the system first during the starting operation. Switch I2I is closed and switches 29 are closed, the contactor initiating the energization of the time delay relay I I9 which closes its contacts I20 a predetermined interval of time after the energization thereof. If it be assumed that it is desired to control the system by means of circuit I23, its switches I20 and I21 are closed. Energization or the actuating coil II I of contactor 20 will not be effected until the contact I30 associated with the voltage divider I25 is' adjusted to the predetermined lower posi tion, effecting closure of contacts I3I and initially biasing the electric discharge control system to a condition which assures a predetermined initial energizatlon of the control winding I4, thereby assuring that a heavy starting load will not be imposed on the motor I. Upon closing contacts I3I, actuating coil H1 is energized effecting operation of contactor 20 which, through its contacts 2I23, connects primary windings I 9 of transformer I8 to the alternating current circuit I6. Upon such operation, the sealing-in contacts I2 Ia are closed so that the adjustable contact I30 in control circuit I23 may be subsequently moved to a position along resistance I25 which pre-establishes the desired speed'oi' the Ian I2. However, th speed responsive part of the control system does not become effective until operation of apparatus described immediately hereinafter.

It will be noted that. by virtue of contacts I03 of contactor 20 during the time prior to the operation of contactor 20, the electric discharge devices and 59 have been maintained at a; predetermined degree of conductivity, as determined by the ratio of resistances III and 95, to lower the potential impressed on control grids 32 01' electric valve means 25-28, inclusive, to a value which limits the amount of current transmitted to the control winding I4.

01 course, the synchronous motor has been started by first connecting the source of reduced voltage 4 to the armature windings 2 through switch 5, and when the speed attains a sufllciently high value, switch 5 is opened and switch 6 is closed thereby connecting the armature winding 2 to the alternating current circuit 1 of normal operating voltage. When the speed of synchronous motor I attains a predetermined value established by the adjustment of the frequency responwhich would impose a severe load or overload on the motor I.

During normal operation of the system, variable amounts of unidirectional current are transmitted to the control winding I4 01' the magnetic clutch 8 by electric valve means 2528 to mainasst nor.

impressed on the control grids 32 by means of the circuit including conductors 43 and it. Of course, the resultant voltage impressed on control grids 32 comprises an alternating component of voltage provided by secondary windings 4B and a unidirectional component of voltage of either positive or negative value determined by the operation of the electric discharge devices 55, 59 or 15, ll. As the magnitude of the voltage produced by control circuit including conductors 43 and M becomes less negative or more positive, the electric valve means 25-48 conduct a greater amount of cur rent, and conversely as the voltage becomes more negative the electric valve means conduct a smaller amount of current.

The operation of the tachometer generator 68 and the control circuit which energizes the circuit including conductors 43 and M will now be considered. During normal ranges of operation, the

' potential difference appearing between conductors 43 and 44 changes in response to the speed of the fan l2 so that upon an increase in speed above a predetermined value or speed level, the output of the electric valve means 25-28 will tend to decrease, thereby permitting a greater slip be tween the members 9 and I ll of the magnetic clutch 8. On the other hand, upon a decrease in speed below the preset value the output current of the electric valve means 25-48 will be increased, thereby decreasing the slip between the members 9 and l of the clutch and tending to restore the speed of the fan i2 to the desired value. The output voltage of the tachometer generator 68, effective through circuit ii and which is proportional to the speed of the fan i2, is measured against a predetermined portion or the standard or reference voltage which is deter mined or established by the glow discharge valves 58 and BI. The amount of this standard Voltage which is preselected by means of the voltage di vider including resistance I25 in circuit E23 and resistor I28 determines the voltage which the tachometer generator 68 must attain in order to control the output of the electric valve means 25-28. The diiierence between the output of the tachometer generator 68 and the predetermined percentage of the glow discharge valve reference voltage is amplified by the control elec tric discharge devices 62 and 63, and the amplified voltage or signal is applied to the grids of discharge devices 55 and 59. Discharge devices 55 and 59 may be considered to constitute the variable impedance arm of a Wheatstone bridge, the other three arms of which are resistance resistance 79, resistance 89, and resistanceill. When the voltage applied to the grids 5E and of discharge devices 55 and 59 is such that the voltag drop across 51 is equal to that across resistance '19 and resistance 83, then the output voltage from the bridge is zero, and so is the unidirectional component of voltage between conductors 43 and 44 so that the output current of the electric valve means 25-2B will be that corresponding to a point within the control range.

If grids 56 and 60 of electric discharge devices 55 and 59 are made less negative, or positive, with respect to their associated cathodes, the potential of conductor Ml will be depressed or lowered, and will be negative in polarity with respect to conductor 43, because discharge devices 55 and 59 will pass more current and the voltage drop across resistance 5? will be greater than that across resistance '59 and resistance 88.

"when the potential of conductor i is negative with respect to conductor the control point of the grids 32 of electric valve means 2528 are lagging in phase by a greater amount and the output from the electric valve means is decreased. Conversely, when the grids and til of discharge devices 55 and 59 are made less positive or more negative, the potential of conductor i iwill rise because discharge devices 55 and will pass less currentjand the voltage drop across resistance 51 will be less than that across resistance l9 and resistance 80. When the potential or conductor 44 is positive with respect to conductor 83, then the grid control point of electric valve means 25-28 is advanced in phase, and the output of the electric valves is increased. Thus, upon an increase in tachometer speed and output voltage above the preset value determined by the adjustment of contact I30 in control circuit lift, the grids of the discharge devices and (it become less positive or more negative, so that these tubes transmit less current and the lower terminal of resistance rises in potential so that the grids of discharge devices 55 and 59 become less negative, or more positive, and discharge devices 55 and 59 transmit more current so that conductor 44 is depressed or lowered in potential, thereby phasing back the grid potentials of the electric valve means 25-28 and decreasing the excitation current transmitted to the control winding It, so the speed of the fan i2 is caused to decrease until an equilibrium condition is reached.

The current limit or preset means 86 serves to limit the armature current of the synchronous motor 2. The current limit control is obtained by causing a unidirectional voltage to appear between conductors db and bi proportional to the armature current of motor i, and to cause electric discharge devices El and to shunt electric discharge devices and and hence, take control away from the latter discharge devices when the armature current attains the value preset by the current limiting means The voltage appearing between conductors 9H and 9! is compared against the reference voltage produced by glow discharge valves 58 and ti, and when the armature current of the synchronous motor tends to exceed a predetermined value this voltage between these conductors will be approximately equal to the standard voltage provided by the discharge values. Under this condition, the of the discharge devices 87 and 88 will to become less negative, or more positive, so tn 2. these discharge devices transmit a greater of current, thereby 'making the voltage *oss resistance 57? greater than that across rice l9 and resistance 80, and consequently or lowering the potential of conductor tiis marmer, the conductivity of the elecvalve means 25-28 is decreased, causing a decrease in the excitation current transmitted to control winding i l to unload the synchronous motor i sui'ficiently to permit its armature cur rent to decrease below the point at which the current will fall ofi to establish an equilibrium condition. Under this equilibrium condition, the voltage appearing between conductors 9 0 and Si is just sufficient to control discharge devices 87 and 83 to furnish exactly the desired coupling excitation to the magnetic clutch 8. When the dischar e devices 8i and 88 are coming into oporation, discharge devices 55 and 53 are being unloaded because of a consequent fall in speed as indicated by the tachometer generator output voltage.

control operations of discharge devices 62, 63,

and 81, 88 are selectively responsive to the speed controlling operation and the armature current preset means 86 or the armature current of the synchronous motor I.

The operation of the current preset means 86 will now be considered in detail when the system is initiated in its operation, that is, when starting the synchronous motor from standstill. During this starting operation, sudden application of load to the synchronous motor i is prevented. Adjustment of resistance "it determines at which value of armature current the transfer from armature current control to speed control becomes effective. The take-over control involving resistors Hi and 95 and contacts m9 of device causes a potent al to ap ear b tween conductors 90 and Si. and appea s in effect as thou h there were an excessive or hi h armature winding current in mot r i. This volta e occurs prior to any such overload current so that discharge devi es 87 and 88 control electric valve means -28 to main ain a low de ree of conductivity thereof. When the contactor 2a is energized. its contact I09 releases this take-over control. provided, of course. that t e fre uency responsive relay H2 is closed. and the voltage appearing between conductors to and at which does not in efiect represent an overload armature current condition but may be considered as representative of a fictitious con ition. is allowed to decrementally decrease until it is sustained by a true or actual armature current condition obtained from the armature current indicating or controlling means comprising the biphase rec- I tillers of the armature current preset means 88.

Adjustment of resistance it sets the value o the voltage between conductors 9t and 91 and in a sense determines at what value of armature current the control shifts into either current limit operation or speed control operation. The ratio of the potential between conductors 9t and 9! to the armature current is adjusted by means of voltage dividers comprising resistances llll and 108 in the two biphase rectifiers. The voltage impressed across conductors 9i) and 9i then represents the armaturecurrent of the motor i.

In the event the synchronous motor l falls out of step, relay i l2 will be opened, and ii contactor 2G is closed will apply a voltage to the circuit including conductors 90 and M which causes electric discharge devices 81 and B8 to reduce the coupling of the clutch 8 or to maintain it at a relatively low value, so that when synchronous motor re-synchronizes and when relay H2 closes, the coupling of the clutch 8 will be applied gradually and not suddenl as the voltage between conductors to and 9| decreases. When this voltage decreases sufiiciently, the control of the system will be taken over by the load responsive portion of the control system.

Because the control system could lower the potential of conductor 44 sufiiciently so that the grids of the electric valve means 2528 would be effective to render these electric valve means completely nonconducting when the speed responsive system indicates a substantial deenergization of the control winding M, the range of the biasing potential impressed on grids 32, that is, the negative valve, is limited to control electric value means 25-28 to act as an inverter, thereby transferring the decrement of electromagnetic energy of winding H to the alternating current circuit 16. The electric discharge devices 16 and ll establish a limit to the-negative potential to which the conductor 44 may be lowered with respect to conductor 43. This controlling eiTect is obtained when conductor ll tends to assume a potential below that level determined by the adjustable contact 82 of resistance 80. When the potential difference between conductors 43 and it attains a predetermined value, electric discharge devices it and I1 conduct current to maintain the potential of conductor 44 at this predetermined maximum negative value in spite of the tendency of electric dischargedevices 55 and 59 or discharge devices 81 and 88 to depress the potential of conductor 44 to a value more negative.

While I have 'shown and described my invention as applied to a particular system of connections and as embody ng various devices diagrammatically shown, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall with in tl-e true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

i. In combination, an alternating current circu t, an alternating current synchronous motor including armature and field windings, starting means for said synchronous motor comprising means for connecting said armature winding to said alternating current circuit, means responsive to the speed of said synchronous motor for 'eiiecting the energization of said field winding when the speed of said motor attains a predetermined value, means driven by said motor and comprising a magnetic clutch including a control winding, a load device driven by said clutch, a second alternating current circuit, means connected between said second alternating current circuit and said control winding and including a. plurality of electric valve means each having a. control member for controlling the current conducted thereby, means energized from said sec-- ond alternating current circuit for energizing a direct current control circuit, a pair of serially connected electric discharge devices connected to be energized from said direct current circuit and each comprising a grid, means for connecting said discharge devices to the control members, means connected to the grid of one of said discharge devices for controlling the conductivity thereof in response to the speed of said load device, means connected to the grid of the other discharge device for controlling the conductivity thereof to cause said plurality of electric valve means to operate as an inverter to transmit energy from said control Winding to said second alternating current circuit when the last mentioned means dictates a reduction in speed of said load device, and current limit means responsive to the armature current of said motor for controlling said one electric discharge device sive to the speed of said synchronous motor for eifecting energization of said field winding when the speed of said motor attains a predetermined value, means driven by said motor and comprising a magnetic clutch including a, control windins, a load device driven by said clutch, a second alternating current circuit, means connected bei tween said second alternating current circuit and said control winding and including a plurality of electric valve means each having a control member for controlling the currentconducted thereby, means energized from said second alternating current circuit for energizing a direct current control circuit, a pair of serially connected electric discharge devices connected to be energized from said direct current circuit and each comprising a grid, means for connecting said discharge devices to the control members, means connected to the grid of one of said discharge devices for controlling the conductivity thereof in response to the speed of said load device, means connected to the grid of the other discharge device for controlling the conductivity thereof to cause said plurality of electric valve means to operate as an inverter to transmit energy from said control winding to said second alternating current circuit when the last mentioned means dictates a substantial reduction in speed of said load device, current limit means responsive to the armature current of said motor for controlling said one electric discharge device and to limit its range of operation when the armature current tends to exceed a predetermined value, and means connected to modify the operation of said last mentioned means during the starting period of said motor for controlling said one discharge device to establish a predetermined maximum degree of conductivity of said plurality of electric valve means.

3. In combination, a source of current, a motor, load means driven by said motor and comprising a control winding, current supply means cohnected to said source for energizing said control winding, circuit controlling means interposed be- ;tween said source and said current supply means for controlling the circuit controlling means and an actuating coil therefor, control connection therebetween, said i means for determining the current transmitted by .said current supply means, means for presetting a condition of said control means, and means actuated by said last mentioned means for controlling the energization of said actuating coil.

4. In combination, an alternating current circuit, an alternating current motor of the synchronous type comprising armature and field windings, starting means for said synchronous motor, load means driven by said motor and comprising a control winding for controlling the load imposed on said motor, electric valve means for variably energizing said control winding, means responsive to the armature current for controlling said electric valve means, and means comprising a source 01 control voltage connected to modify said last mentioned means independently of variations of said armature current for controlling the conmeans comprising switchingductivity of said electric valve means to limit the maximum conductivity thereof during the starting operation.

5. In combination, an alternating current circuit, a synchronous motor having armature and field windings, starting means for said motor comprising means for connecting said armature winding to said alternating current circuit, means responsive to the speed of said motor for effecting energization of said field winding, load means driven by said motor, means for controlling the load imposed on said motor and comprising a control electric discharge device, a source of control potential of fixed value suflicient to render said discharge device conductive, and means responsive to the speed responsive means for impressing said control potential upon said discharge device during subsynchronous operation of said motor.

6. In combination, an alternating current circuit, a synchronous motor having armature and field windings, starting means for said motor, load means driven by said motor and comprising a control winding, electric valve means for variably energizing said control winding, means responsive to the speed of said synchronous motor during the starting operation for energizing the field winding when the speed attains a predetermined value, means for providing a fixed component of control voltage and a component of control voltage variable in accordance with the current of said armature winding, armature current preset means jointly responsive to said components of voltage for controlling said electric valve means to limit the load imposed on said motor and for limiting the current transmitted to said armature winding, control means responsive to a predetermined operating condition of said load means, and means selectively responsive to the armature current preset means and said control means for controlling the conductivity of said electric valve means.

7. In combination, an alternating current circuit, a synchronous motor having armature and field windings, the armature winding being connected to said alternating current circuit, starting means for said synchronous motor, load means driven by said motor and comprising a control winding, means for variably energizing said control winding to control the load imposed on said motor, and voltage control means connected to modify the operation of said last mentioned means for initiating energization of said control winding at a value of energization substantially lower than its maximum operating limit.

8, In combination, an alternating current circult, a synchronous motor having armature and field windings, starting means for said synchronous motor comprising means for connecting the armature winding to said alternating current circuit, load means driven by said motor and comprising a control winding for controlling the load imposed on said motor, means for variably energizing said control winding, means for selectively energizing and deenergizing the last mentioned means, control means for the means which energizes said control winding, and voltage control means connected to modify the energization of said control means upon initiation of the energization of said control winding for efi'ectng only relatively small values of current change therein so that the application of load to said motor is effected gradually.

9. In combination, an alternating current circuit, a synchronous motor having armature and contact which permits for energizing the field winding, load means driven by said motor and comprising a control winding for determining the load imposed on said motor, means for transmitting variable amounts of current to said control winding comprising electric valve means of the controlled type having a control member for controlling the conductivity thereof, control means including a source of voltage for applying a variable voltage to said control member, and means for initiating the energization of said control winding comprising means including a second source of voltage for superimposing upon the voltage of said control means a different voltage of a value to establish a predetermined initial conductivity of said electric valve means lower than the conductivity established by said control means separately,

10. In combination, an alternating current circuit, a synchronous motor having armature and field windings, the armature winding being connected to said circuit, means for energizing the field winding, load means driven by said motor and comprising a control winding, means for energizing said control winding comprising electric valve means having a control member, means responsive to a predetermined operating condition of said load means for controlling the conductivity of said electric valve means and comprising an electric discharge device, and means for initiating energization of said control winding comprising a voltage divider connected to said last mentioned means to provide a component of voltage so as to modify the operation of said discharge device for pre-establishing the value of said electrical condition which is to be maintained and for permitting initiation of the energization of said control winding at only a relatively low value of 'current so that the load is gradually applied to said motor.

11. In combination, an alternating current circuit, a synchronous motor having armature and field windings, the armature winding being connected to said circuit, means for energizing the field winding, load means driven by said motor and comprising a control winding, means for energizing said control winding comprising electric valve means having a control member, means responsive to a predetermined operating condition of said load means for controlling the conductivity of said electric valve means and comprising an electric discharge device, and means for initiating energization of said control winding comprising a voltage divider connected to said electric discharge device and comprising an adjustable contact provided with an auxiliary energization of said electric valve means only when a predetermined degree of conductivity is preestablished.

12. In combination, an alternating current circuit, a synchronous motor having armature and I field windings, the armature winding being connected to said circuit, means for energizing the field winding, load means driven by said motor and comprising a control winding, means for energizing said control winding comprising electric valve means having a control member, means responsive to a predetermined operating condition of said load means for controlling the conductivity of said electric valve means and comprising an electric discharge device, a second electric discharge device connected in parallel with the first mentioned electric discharge device, and armature current limiting means connected to said second electric discharge device for rendering said second electric discharge device conductive thereby rendering ineffective the first mentioned electric discharge device.

13. In combination, an electric circuit, a motor connected to said circuit, load means driven by said motor and comprising a control winding, an alternating current circuit, means comprising electric valve means for variably energizing said control winding, said electric valve means including a control member, means energized from said alternating current circuit for energizing a direct current circuit and comprising rectifying means, a pair of electric discharge devices energized in series relation from said direct current circuit, means for controlling the conductivity of one of the discharge devices in response to a predetermined operating condition of said load means, and means for controlling the conductivit of the other discharge device to cause said electric valve means to operate as an inverter t transmit energy from said control winding to said alternating current circuit when the last mentioned means dictates a reduction in the energization of said control winding.

14, In combination, an alternating current supply circuit, an alternating current synchronous motor comprising an armature winding connected to said supply circuit and including a field winding, means for starting said synchronous motor, load means connected to said motor and comprising a control Winding for controlling the load imposed on said motor, means for variably energizing said control winding comprising electric valve means, means for controlling said electric valve means in response to a predetermined condition derived from said load means, means responsive to the motor armature current and effective to supersede the control effected by said fourth mentioned means during the starting operation for limiting the current supplied to said control winding by said electric valve means, means for initiating energization of said field winding, and means for rendering said control from the current responsive means and said fourth mentioned means simultaneously effective in response to the operation of said field winding energizing means.

15. In combination, an alternating current supply circuit, an alternating current motor of the synchronous type comprising an armature winding and a field winding, said armature winding being'connected to said alternating current circuit, means for starting said motor, load means driven by said motor and including a control winding, means for variably energizing said control winding comprising electric valve means, means responsive to a predetermined operating .condition of said load means for controlling said electric valve means, means for initiating energization of said field winding, means responsive to the armature current of said motor and dominating the control of said load means response during subsynchronous speed of said motor for limiting the energization of said control winding during the starting operation, and means responsive to the field initiating means for transferring the control of said electric valve means from current response to load means response.

16. In combination, an alternating current circuit, an alternating current motor of the synchronous type comprising an armature winding and a field winding, said armature winding being connected to said alternating current circuit, means for energizing said field winding, load means driven by said motor and including a control winding, electric valve'means connected to a source of alternating current for variably energizing said control winding and comprising a control member for controlling the current conducted thereby, means responsive to a predetermined operating condition of said load means for 'controlling the potential impressed on said conof said motor for transferring the control of said electric valve means from response to said load means to armature current response when the armature current attains a predetermined value.

17. In combination. In alternating current suppl circuit, a dynamo-electric machine of the synchronous type energized from said supply circuit, a load device, means connected between said machine and said load device and including a magnetic clutch comprising a pair of relatively rotatable members one of which includes a control winding for determining the relative rotation of said members, means for variably energizing said control winding comprising electric valve means, means responsive to the speed or said load device for producing a control voltage continuously variable in proportion to said speed, and means for controlling said electric valve means in accordance with said control voltage.

ELMO E. MOYER. 

